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MIMO Time Reversal Communications

MIMO Time Reversal Communications. Hee-Chun Song, W.S. Hodgkiss, and W.A. Kuperman Marine Physical Laboratory/Scripps Institution Oceanography University of California, San Diego 9500 Gilman Drive, La Jolla September 14, WUWNet 2007, Montreal, Canada. Outline.

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MIMO Time Reversal Communications

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  1. MIMO Time Reversal Communications Hee-Chun Song, W.S. Hodgkiss, and W.A. Kuperman Marine Physical Laboratory/Scripps Institution Oceanography University of California, San Diego 9500 Gilman Drive, La Jolla September 14, WUWNet 2007, Montreal, Canada

  2. Outline • Underwater Acoustic (UWA) Channel • Time reversal mirror (TRM) • Application to communications • Multi-user communications: Experiments • Downlink (active) • Uplink (passive) • Summary

  3. Radio vs. Underwater Acoustic Channel

  4. Challenges in UWA Channel • Delay Spread (ISI) • Doppler Spread Stojanovic et al., “Phase-coherent digital communications for underwater acoustic channels,” IEEE J. Oceanic Eng., 1994. Commercial modem: Incoherent FSK (Frequency shift keying) Research community: Multi-channel DFE with 2nd order PLL

  5. BACKGROUND: TRM • CLAY AND PARVELESCU (underwater acoustics) 1965 • ZEL’DOVICH (non-linear optics): Phase Conjugation 1972 • Retrograde antenna (microwave) 1964 • BUCKER (underwater acoustics - mfp) 1976 • TAPPERT et al. (underwater acoustics - “retrograde …”) • FINK et al. (ultrasonics): Laboratory experiment1989 • JACKSON AND DOWLING (underwater acoustics) 1991,1992 • Marine Physical Laboratory and SACLANTCEN (implementation of TRM in ocean) 1996 - present http://www-mpl.ucsd.edu/people/wak

  6. FAF-99: Experiment @ 3.5 kHz VRA SRA (TRM)

  7. Multi-focus

  8. 3.5 kHz SRA (’99 and ’00) L = 78 m N = 29 3.5 kHz tranceiver

  9. Application to Acomms Probe Source Pulse Normal Time-Reversal Time-Reversal Communications Self-equalization ACTIVE (downlink)

  10. TR: Self-Equalization Process • Channel complexity (number of multi-paths) • Number of array elements • Spatial distribution (spacing and aperture) TR + Equalization • Remove the residual ISI • Compensate for time variations in the channel

  11. Performance Comparison: Theory Channel Model TR+EQ TR+EQ TR OutputSNR Stojanovic, JASA (2005) Song et al., JOE (2006) Song et al., JASA (2007) Input SNR

  12. FAF04: Active Time Reversal + DFE TRM+DFE TRM Only SNR0=13.7 dB SNR0=26.3 dB Song et al., IEEE JOE (2006)

  13. SIMO PASSIVE ACTIVE MISO Song et al., JASA (2006)

  14. Multi-channel DFE w/ implicit matched filter Time Reversal Passive time reversal Followed by a single Channel DFE Yang, JOE (2005) Correlation-based DFE

  15. Multi-User MISO(Active Time Reversal) Downlink Song et al., IEEE JOE 31, 2006

  16. MIMO Implementations: Round Robin

  17. FAF03: MISO 3-User Acomm (QPSK) Range = 8.6 km 29.7 dB 27.7 dB 25.5 dB

  18. Multi-Access SIMO(Passive Time Reversal) Uplink Song et. al., JOE (2007)

  19. FAF05: SIMO Multi-user Acomms(3-4 kHz)

  20. FAF05: Multi-Access SIMO, R=4 km Co-channel interference LFM Receiver

  21. FAF05: Single User SIMO, R=4 km Single User ISI: 45 symbols 96-m

  22. FAF05: Multi-Access SIMO, R=4 km 3 Users 6 Users

  23. Co-Channel Interference 6 Users

  24. FAF05: Multi-Access SIMO, R=20 km(SRA-VRA2)

  25. CIR Comparison @ R=4 and 20 km R=4 km, ISI: 45 symbols R=20 km, ISI: 10 symbols Use bottom 20 elements for processing

  26. FAF05: Multi-Access SIMO, R=20 km 3 Users 3 Users 1 User

  27. Summary • Time reversal mirrors, either active or passive, exploit spatial diversity to achieve temporal and spatial focusing in complex environments (e.g., acoustic waveguide) • Temporal focusing mitigates ISI while spatial focusing enables a straightforward extension to multi-user MIMO communications. • Active (downlink) and passive (uplink) time reversal is equivalent mathematically with the communications link being in the opposite direction. • Time reversal approach provides optimal performance in theory when combined with adaptive channel equalization. • At-sea experimental results in shallow water have demonstrated the effectiveness of time reversal communications.

  28. Performance Prediction: FAF05, QPSK Song et al., JASA (2007) M=2 Fractionally-sampled DFE Theory: Stojanovic, JASA (2005)

  29. Time Reversal Communications(11-19 kHz)

  30. FAF06: f =15 kHz, W=7.5 kHz ISI:100 symbols ISI:120 symbols 16 kbits/s at 2 km range 15 kbits/s at 5 km range

  31. Passive Time Reversal + DFE

  32. Array Resolution: Free Space L R Rayleigh diffraction limit

  33. Array Resolution: Waveguide 1 V(q) qc qe B S B S B S Le Lc B qe S B S qc B S

  34. 0 -5 -10 Amplitude -15 -20 -25 -30 -10 -5 0 5 10 Distance (mm) Spatial focusing of the time reversed wave Mobile hydrophone Time reversed signals Fink et al

  35. Single Channel Communication w/ a Moving Source T = 1-ms, 3 kHz, N=9002 symbol LFM, 100-ms, 2-4 kHz

  36. Time Reversal vs Equalizer • MLSE • MMSE • LE • DFE Song et al., IEEE JOE 31, 2006 Song et al., JASA 120, 2006 Yang, JOE 29, 2004

  37. FAF04: Pianosa Island (SW of ELBA) Shallow Water Experiment Depth: 50 m Range: 2 km

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